BUG: Fix linkage for SmartPointerForwardReference for clang 4

Unifying SmartPointerForwardReference instantiation into a single
file.

This patched addressed linking issues with clang 3.0 when compiling
with shared libraries. Addresses the following linking error:

Undefined symbols for architecture x86_64:

  itk::SmartPointerForwardReference<itk::ProcessObject>::operator
  itk::ProcessObject*() const", referenced from:

Change-Id: Ib6d84681e677b47722c26a2ed698f1018c2b17a7